The destruction of bacterial spores occurs in two operations which are very different from each other in their aims:
Sterilization with a view to destruction of all the bacterial spores present with a degree of reliability compatible with the commercial requirements for sale of the sterile product distributed in sterile apparatus. An example of this is the sterilization of various milk processing equipment.
Decontamination of lower the spore count to a level considered to be acceptable for the subsequent use of the product. Such an operation is carried out, for example, in certain areas of the vegetable processing industry. Decontamination may precede sterilization in cases where pieces or sets of pieces are difficult to sterilize in situ.
In the context of the present invention, it should be understood that although the process described below is more particularly intended for sterilization, it is equally or even more applicable to the decontamination of apparatus and materials.
A sterilization treatment uses a sporicidal agent for a given time at a given temperature.
By "sterilization parameters" are meant the combination of time and temperature which characterises a given process and produces a given sterilisation effect.
Calculation of the parameters is based on precise knowledge of the treatment conditions, in particular the pH, temperature and activity of the water, and of the kinetics of destruction of the bacterial spores under these conditions. To make the calculation, the data relating to bacterial spores of the most heat-resistant strains which it is desired to destroy are used. These strains, (or strains with similar properties), should normally be present in the product or on the material to be sterilized and must be capable of developing in the product after sterilization.
The sterilization parameters can only be calculated if the destruction kinetics of the bacterial spores obey a constant law which is reproducible from one treatment to the next. Sterilization by heat has a considerable advantage from this point of view. The destruction kinetics of bacterial spores may generally be represented by the following equation: EQU E=log (N.sub.o /N)=t/D.sub.t ( 1)
where E, the sterilization effect (defined as the logarithm of the ratio of N.sub.o, the initial number of bacterial spores, to N, their number after a destruction treatment of duration t) is the ratio of the treatment time t to a constant D.sub.T characteristic of the treatment and the strain, also known as the decimal reduction at temperature T. The destruction kinetics can therefore easily be represented by a straight line obtained by plotting log N as a function of t.
Unfortunately, many sporicidal treatments exist in which the destruction kinetics do not obey such a simple law. These include in particular treatment with chemical substances, where the kinetics of destruction are represented by a "trail" or scattering of points: "biphasic" kinetics or kinetics characterised by an upward concavity (in both cases on semilogarithmic coordinates as mentioned above). To the extent that the reproducibility of such kinetics from one test to another is poor, it is difficult, if not impossible, to calculate the parameters for a sporicidal treatment which has the serious disadvantage described above.
Numerous examples of destruction kinetics which cannot be described by equation (1) are found in the following publication: "The trail of survival curves of bacterial spores" by O.CERF, Journal of Applied Bacteriology (1977) 42: 1-19.
The cases of treatments with an alkaline medium, or with methylene glycol, glutaraldehyde, hydrogen peroxide, or ethylene oxide may be briefly mentioned here.
It should be clear that the complete study of a sterilizing agent is a very prolonged and costly operation. This partly explains why, in the case of many chemical additives, considerable difficulties were encountered when these studies were conducted with insufficient thoroughness. It is one of the advantages of the process according to the present invention that the studies which have already been carried out on sterilization by moist heat can to a large extent be used again.
The process according to the present invention uses a heat effect additive which makes it possible for the known curves for moist heat sterilization to be used but under more favourable conditions of temperature and pressure.
Although sterilization with moist heat may be regarded as a sterilization process which produces completely satisfactory results, there are nevertheless certain disadvantages in the methods employed.
Sterilization with moist heat implies the use of temperatures above 100.degree. C. It is well known that each degree of temperature above 100.degree. C. involves a high additional cost. It is therefore of particular interest to be able to achieve sterilization at temperatures below 100.degree. C.
Furthermore, sterilization with steam or with hot water under pressure means that the whole apparatus which is to be contaminated by this process must be capable of withstanding the extra pressure. In the majority of cases this means the use of a technology which requires very costly apparatus. It is therefore particularly important to be able to operate under conditions as close as possible to atmospheric pressure. Particularly in processes for the ultra high temperature sterilization of milk, for example, this means that a simple buffer vat may be used instead of an autoclave, and ordinary heat resistant joints may be used instead of pressure resistant joints.